The present invention relates to a sensor signal processing apparatus for extracting a signal from a sensor section as a function of physical quantity to be measured and, more particularly, to a sensor signal processing circuit for eliminating errors originating from elements due to temperature changes and the like (these errors will be referred to as element-based errors hereinafter) and errors originating from a circuit due to the offset of an operational amplifier and the like (these errors will be referred to as circuit-based errors hereinafter) by signal processing.
Recently, in the field of pressure measurements, electronic pressure gages have been rapidly replacing mechanical pressure gages. The electronic pressure gages can be roughly classified into a resistance type that converts a pressure change in a pressure-sensitive diaphragm into an electric resistance change and a capacitance type that converts a displacement of a pressure-sensitive diaphragm into a capacitance change. Of these types of sensors, a capacitance type pressure sensor is excellent at fine pressure measurement.
FIG. 16 shows the structure of the above capacitance type pressure sensor. Referring to FIG. 16, a first recess portion 101a is formed in the central portion of the surface of a pedestral substrate 101. A second recess portion 101b is formed in the shape of a groove to surround the first recess portion 101a through a barrier 104. A thin diaphragm substrate 102 is joined to that surface of the pedestral substrate 101 on which the recess portions 101a and 101b are formed. The spaces surrounded by the first and second recess portions 101a and 101b and diaphragm substrate 102 on the pedestral substrate 101 form capacitor chambers 103a and 103b. 
As shown in FIG. 17, a square fixed electrode 105a is formed on the bottom surface of the first recess portion 101a. A movable electrode 105b is formed on the diaphragm substrate 102 to oppose the fixed electrode 105a at a predetermined distance therefrom. The electrode 105a is extracted to the outside by a lead 105c. The electrode 105b is also extracted to the outside by a lead (not shown). A sensor capacitor 114a (FIG. 18) to be described later is constituted by the pair of electrodes 105a and 105b and the air existing between the electrodes 105a and 105b and serving as a dielectric member.
As shown in FIG. 17, a belt-like electrode 106a is formed in the shape of a square frame on the bottom surface of the second recess portion 101b. An electrode 106b is formed on the diaphragm substrate 102 to oppose the electrode 106a at a predetermined distance therefrom. The electrode 106a is extracted to the outside by a lead 106c. The electrode 106b is also extracted to the outside by a lead (not shown). A reference capacitor 114b (FIG. 18) to be described later is constituted by the pair of electrodes 106a and 106b and the air existing between the electrodes 106a and 106b and serving as a dielectric member.
Note that the barrier 104 between the capacitor chambers 103a and 103b is partly removed to allow the air in the capacitor chambers 103a and 103b to easily mix.
A portion of the diaphragm substrate 102 which is part of the capacitor chamber 103a serves as a pressure-sensitive diaphragm 102a. As shown in FIG. 16, therefore, when a positive pressure P is externally applied to the diaphragm substrate 102, the pressure-sensitive diaphragm 102a deflects toward the capacitor chamber 103a. Since the electrode 105b is displaced as the pressure-sensitive diaphragm 102a deflects, the gap between the electrodes 105a and 105b decreases, and a capacitance Cs of the sensor capacitor 114a increases. At this time, a portion of the diaphragm substrate 102 which is part of the capacitor chamber 103b does not deflect upon application of the pressure P, and hence a capacitance Cr of the reference capacitor 114b does not change. That is, the sensor capacitor 114a functions as a first sensor element whose capacitance Cs changes in accordance with a change in the pressure P.
That portion of the diaphragm substrate 102 which is part of the capacitor chamber 103b does not deflect upon application of the pressure P because the capacitor chamber 103b is narrow. For this reason, the capacitance Cr of the reference capacitor 114b does not change. That is, the reference capacitor 114b functions as a second sensor that exhibits the constant capacitance Cr even with a change in the pressure P.
The reference capacitor 114b is formed to eliminate measurement errors (element-based errors) due to temperature changes around a sensor section 114, humidity changes in the capacitor chamber 103a, and the like. More specifically, the pressure P from which the above measurement errors are eliminated can be theoretically obtained by calculating
K1=(Csxe2x88x92Cr)/Csxe2x80x83xe2x80x83(1)
on the basis of the capacitance Cs of the sensor capacitor 114a and the capacitance Cr of the reference capacitor 114b. 
Letting ∈ be the dielectric constant of the air in the capacitor chambers 103a and 103b, d be the gap between the electrodes 105a and 105b in the sensor capacitor 114a (in non-measurement period) and the gap between the electrodes 106a and 106b in the reference capacitor 114b, xcex94d be the pressure sensitivity displacement of the pressure-sensitive diaphragm 102a, and S be the area of each of the opposing surfaces of the electrodes 105a and 105b and the areas of the opposing surfaces of the electrodes 106a and 106b for the sake of simplicity, the capacitances Cs and Cr can be given by
Cs=∈S/(d+xcex94d)xe2x80x83xe2x80x83(2)
Cr=∈S/dxe2x80x83xe2x80x83(3)
Substitutions of equations (2) and (3) into equation (1) yield
K1=xe2x88x92xcex94d/dxe2x80x83xe2x80x83(4)
Obviously, therefore, the pressure P can be obtained from equation (1).
FIG. 18 shows a sensor signal processing circuit for extracting a signal from the sensor section 114 in FIG. 16 as a function of the pressure P.
Referring to FIG. 18, the input side of the sensor capacitor 114a of the sensor section 114 is connected to an AC power supply 111 through a buffer 113a and switching section 112. The input side of the reference capacitor 114b is connected to the AC power supply 111 through a buffer 113b and the switching section 112. An amplifying section 115 is connected to the output side of the sensor section 114. A CPU (Central Processing Unit) 117 is connected to the output side of the amplifying section 115 through an A/D (Analog-to-Digital) converter 116.
The amplifying section 115 is comprised of an operational amplifier 115a and capacitor 115b. The noninverting input terminal (+), inverting input terminal (xe2x88x921), and output terminal of the operational amplifier 115a are respectively connected to the ground (G), a node 114c of the capacitors 114a and 114b, and the A/D converter 116. The capacitor 115b is connected to the node 114c of the capacitors 114a and 114b and the output terminal of the operational amplifier 115a. 
The CPU 117 outputs control signals 118 for switching operation to the switching section 112, and performs arithmetic processing upon combining signals output from the A/D converter 116 for every switching operation of the switching section 112.
Letting Vi be the output voltage from the AC power supply 111, and Cf be the capacitance of the capacitor 115b, an output voltage V101 from the amplifying section 115 when the power supply 111 is connected to the sensor capacitor 114a can be given by
V101=xe2x88x92CsVi/Cfxe2x80x83xe2x80x83(5)
An output voltage V102 from the amplifying section 115 when the power supply 111 is connected to the reference capacitor 114b can be given by
V102=xe2x88x92CrVi/Cfxe2x80x83xe2x80x83(6)
Therefore, Kl expressed by equation (1) can be obtained by using equation (7) below:
(V101xe2x88x92V102)/V101=(Csxe2x88x92Cr)/Cs=K1xe2x80x83xe2x80x83(7)
In the signal processing circuit shown in FIG. 18, however, the relations expressed by equations (5) and (6) cannot be properly obtained because of wiring capacitances, offsets of the buffers 113a and 113b and operational amplifier 115a, and the like. More specifically, letting e101, e102, and e103 be errors based on the offsets of the buffers 113a and 113b and operational amplifier 115a, the output voltages V101 and V102 from the amplifying section 115 are given by
V101=xe2x88x92Cs(Vi+e101)/Cf+e103xe2x80x83xe2x80x83(5a)
V102=xe2x88x92Cr(Vi+e102)/Cf+e103xe2x80x83xe2x80x83(6a)
As indicated by equations (5a) and (6a), the errors e101 to e103 cannot be eliminated. The same applies to wiring capacitances. For this reason, only measurement results containing circuit-based errors can be obtained.
In addition, in the conventional sensor signal processing circuit, since equation (1) cannot be obtained from equations (5a) and (6a), element-based errors due to temperature changes and the like cannot be eliminated.
According to the conventional sensor signal processing circuit, therefore, an accurate, high-precision measurement result cannot be obtained because both circuit- and element-based errors are contained in the result.
It is an object of the present invention to provide a sensor signal processing apparatus designed to improve the measurement precision of a sensor.
In order to achieve the above object, according to the present invention, there is provided a sensor signal processing apparatus comprising sensor means whose characteristics change in accordance with a change in physical quantity to be measured, power supply means for supplying powers of two systems having different polarities to the sensor means, switching means, connected between the power supply means and the sensor means, for switching combinations of powers of the two systems from the power supply means while preventing mixing of powers of the two systems, and arithmetic means for obtaining a ratio between differences between a plurality of signals output from the sensor means for every switching operation of the switching means.